1. Field of Invention
This invention relates generally to the optical pumping and techniques for cooling a solid state crystal laser medium and more particularly to passively cooling the laser medium by the removal of heat by heat conductive elements in heat conductive relationship with the laser medium to produce an isotropic temperature condition in the laser medium.
2. Discussion
High intensity, high power solid state laser systems that are optically pumped generate a great deal of heat within the laser slab. Unless the temperature is controlled by appropriate techniques that are consistent with the application environment, the systems can be very inefficient and even inoperative. For example, when such laser systems are used in outer space, the cooling techniques that can reasonably be used are limited. It is known in the case of terrestrial applications for such systems to use fluids as the cooling medium. Fluids as coolants require circulating pumping equipment particularly if water is used; or in the case of air the use of blowers. In any event, such use of fluid coolants present the risk of contaminating the laser medium surfaces which could interfere with the quality of the laser beam that is generated. Additionally, fluids can freeze in certain environments, damaging the hardware.
As is well-understood in this art, beam quality of these systems is a function of the uniformity of the temperature throughout the laser medium as opposed to the absolute temperature of the medium. Cooling of the medium is directed at creating an isotropic condition so that as the generated laser beam is propagated through-the laser medium the wave form will have a uniform phase across the entire beam. It is the temperature variation encountered in the laser medium which varies the refractive indices encountered in its path causing it to distort. This difference in phase across the output beam is known as the optical path difference ("OPD").
As the beam experiences a certain amount of divergence its beam quality is inversely proportional to the level of the OPD. This relationship can be expressed mathematically: ##EQU1## It will be appreciated that small increases in OPD, which is taken as the root mean square of the amplitude of the wave form, will result in a significant degradation of the beam quality. Understandably the design and construction of laser systems has as one of its primary objectives to provide an isotropic temperature profile within the laser medium. Another factor that affects OPD is the geometric relationship of the direction of the optical pumping and temperature control elements applied to certain face portions of the lasing medium. Generally the lasing medium has opposing polished faces and opposing roughened face portions that diffuse incident radiation. With respect to the geometry of pumping and cooling the laser it has been found that pumping the laser from only one surface and cooling it from another opposite surface results in very poor beam quality and is not acceptable. Improved beam quality results when the geometry is symmetrical and involves optical pumping from opposite faces of the medium and the cooling path is also imposed on opposing faces and transverse the optical pumping path. This is known as a two-sided symmetrical pumping and two-sided cooling. The OPD for such a geometry of pumping and cooling will result in greatly improved results in the beam quality.
The manner of cooling the laser medium plays a significant role in determining beam quality. Previously known techniques employed heat exchangers using water or other suitable fluids to flow over the face portions to remove the heat. As a heat exchanger it required pumping equipment to recirculate the liquid over the face portion. The opportunities for contamination of the liquid presented the problem of depositing contaminants on the laser face portion which interfere with the internal reflection of the incident beam inside the face. Such known disadvantages of heat exchanger techniques are characteristic of convective cooling. Convective cooling techniques with respect to the description of this invention are defined as employing pumping equipment and recirculating liquids and gases directly across the face portions to remove the heat. This invention employs conductive cooling techniques which employ thermally conductive layers of materials placed against the cooling face portions for conductively transferring the heat away from the laser medium absent of coolant passing over the laser medium.
As described earlier, the significant advantage of conductive cooling techniques obviates the problem of contaminating the face portions of the laser medium when using fluid exchangers for cooling. The advancement in the use of conductive cooling as described herein allows cooling of the laser medium without the use of circulating fluids and does not require the use of circulating pump equipment. Conductive cooling also affords the opportunity of being able to use liquid type heat exchange systems but eliminates the risk of introducing contaminants to the faces of the laser medium. The invention also provides for the use of alternative types heat exchangers which are completely divorced from the faces of the heat exchanger being affixed to the conductive assembly to effectively dissipate the heat without the risk of contamination.